When cycling through [closely aligned] faces on a computer screen, each face seems to become a caricature of itself and some faces appear highly deformed, even grotesque. The degree of distortion is greatest for faces that deviate from the others in the set on a particular dimension (eg if a person has a large forehead, it looks particularly large).

Watching the video up top should illustrate the illusion quite nicely. It's not unlike viewing the world through a fun-house mirror that lives in your mind.

What's cool about this illusion is that it was discovered completely by accident. Murphy had been working alone in the lab, skimming through photographs of faces aligned at eye-level, when he noticed that some of the faces lurking on the fringes of his vision looked frightening and deformed. The instant he tried to get a closer look at them, however, they appeared totally normal — attractive even. Can you imagine working in the lab late one night, only to discover a novel optical illusion that tricks you into perceiving attractive faces as utterly grotesque? Don't pretend for a second that wouldn't freak you the hell out.